The present invention relates generally as indicated to offshore electronic surveys for marine pipelines, and more particularly to a method and apparatus for making closely-spaced electrical survey measurements for marine pipelines and the like.
Pipelines or other elongate structures in marine environments are normally installed with some form of cathodic protection system to prevent premature corrosion failures of the pipelines. An electrical current is usually passed through the water from anodes placed adjacent to a specially coated pipeline along the length thereof to reduce the potential difference between the environment and pipeline to a minimum negative potential difference. Two basic systems are employed. In galvanic protection systems, sacrificial anodes are placed in close proximity to the pipeline at spaced intervals along the length thereof. Such anodes are self-energized and are usually connected directly to the pipeline to be protected. The other basic system, commonly referred to as impressed current type, employs anodes energized by a direct current power supply such as a rectifier. The anodes are connected directly to the positive terminal of the rectifier, with the pipe or structure being protected connected to the negative terminal.
Failure of cathodic protection systems in a marine environment may occur due to inadequate design, coating damage, anode passivation, premature anode depletion, inadequate current distribution and improper anode placement. Underwater pipelines also may shift causing or aggravating the above problems. Repair of the cathodic protection system is possible, but such repair should be made before irreperable damage occurs to the structure or pipeline, as otherwise, more expensive repairs of the pipeline will be necessitated. To maintain reliability and insure optimum efficiency of the cathodic protection system, evaluations through electrical measurements are required to detect damage to the system so that necessary repairs can be made.
Such electrical corrosion measurements may be made by placing a reference electrode, such as a silver-silver chloride half cell, proximate the pipeline and measuring the potential difference between the reference electrode and pipeline. Since these electrical measurements require a contact to the pipeline, heretofore, such measurements were made on shore or on offshore platforms where the pipeline is readily accessible. In addition, very costly diver assisted measurements have been made at sub-sea valves or at relatively far apart test stations. Accordingly, areas of substantial corrosion damage may go undetected leading to premature failure of the pipeline.